2~~OR68
1.
"METHOD FOR REMOVING THE ACIDIC IMPURITIES CONTAINED
IN A VAPOURIZED STREAM COMPOSED BY ORGANIC VAPOURS AND
41ATER"
The present invention relates to a method for
removing the acidic impurities contained in a
vaporized stream consisting of organic vapours and
water.
More particularly, the present invention relates
to a method for removing the acidic impurities
contained in a vaporized stream consisting of an
alkanol, an alkyl carbonate, water and still other
organic species in minor amounts, which removal method
consists in sending said stream to an adsorption
reactor containing a solid adsorbent bed comprising
alumina or activated charcoal.
A further object of the present invention is the
application of said removal method to the removal of
acidic impurities, more precisely hydrogen chloride,
from a vaporized stream mainly containing methanol,
dimethyl carbonate , water and minor amounts of other
organic species, in a process for dimethyl carbonate
synthesis.
Dimethyl carbonate ("DMC" herein in the
following) is a widely used, very flexible chemical
product which is used as such as solvent, or as a fuel
additive; furthermore, DMC is an important
intermediate product in the synthesis of other alkyl
or aryl carbonates, useful as synthetic lubricants,
solvents, monomers for polymeric materials and for
preparing isocyanates, urethanes, ureas and
~mosss
2.
polycarbonates.
The most widely used route for producing DMC
presently is the one which is based on oxidative
methanol carbonylation, in particular in the presence
of CuCI as a catalyst, according to the reaction
equation:
2 CH30H + CO + 1/2 02 ----> (CH30)2C0 + HZO
The preparation of DMC according to such a
reaction is disclosed, e.g., in commonly owned U.S.
patents 4,218,391 and U.S. 4,318,862, and in European
patent applications EP-A-460,732 and EP-A-460,735.
In the processes which are based on DMC
preparation method as reported above, the separation
of reaction effluent from catalyst is usually
advantageously carried out by evaporation, by taking
advantage of the difference in volatility between the
organic products and water on the one hand (volatiles)
and the components of the catalytic system (non-
volatiles) on the other hand. In that way, vapour
streams are obtained which typically contain (% by
weight):
-- CH30H: from 45% to 70%;
-- DMC: from 25% to 50X;
-- H20: from 2X to 6%; and
-- other organic byproducts: from 2% to 3%.
Unfortunately, the above said streams result to
be contaminated by variable amounts of hydrogen
chloride, because the halogen is an essential
component of the catalytic system. According to the
adopted working methods, such amounts can be usually
210868
3.
comprised within the range of from 5 ppm to 600 ppm of
hydrogen chloride, by weight, based on weight of
vapours.
The presence of chloride ions causes considerable
problems of technical-economic nature in the facility
section downstream of the reactor, i.e., that section
of the facility in which the product separation and
purification operations are carried out.
In fact, the presence of hydrogen chloride causes
very serious problems of equipment corrosion, with the
need consequently arising for having resort to
special, corrosion resistant material for building the
separation and purification sections and with a
considerable burden.
The removal of chloride ions contained in the
vaporized phase effluent from dimethyl carbonate
synthesis reactor would therefore allow the above
disclosed drawbacks to be overcome.
According to the techniques known from the prior
art, the removal of acidic impurities, in particular
hydrogen chloride, contained in a stream of organic
vapours, can only be carried out after condensing said
stream, so as to obtain a liquid phase.
After obtaining the liquid phase, the removal of
the acidic impurities, in particular hydrogen
chloride, can be carried out by treating said liquid
phase with an alkaline or basic agent, either in solid
state, or in solution (neutralization). Unfortunately,
in this case, a large number of problems arise which
are associated with the precipitation of salts formed
21~08fi8
4.
during the neutralization reaction in the operating
facilities, with consequent fouling of the latter;
with their separation from the process fluids and
their end disposal to dump; with the decomposition of
produced DMC induced by alkaline hydrolysis in case of
feed of an excess amount of the alkaline or basic
agent.
According to an alternative route, the liquid
phase obtained from the condensation of the stream of
organic vapours, can be caused to flow above a fixed
bed constituted by ionic exchange resins of cationic
type. Unfortunately, also in this case, problems arise
from the absorbent power of ion exchange resins of
cationic type which is generally rather low, it being
comprised within the range of from 4 to 5 equivalents
per kg of dry resin.
The present Applicant has now found a method
which makes it possible the acidic impurities to be
removed from a vaporized organic phase, by directly
operating in the vapour phase, and thus avoiding the
problem of condensation of the vaporized phase into a
liquid phase, in a simple and advantageous way, by
using solid adsorbent beds comprising alumina or
activated charcoal. Such a method makes it possible
the level of said acidic impurities to be reduced down
to such values -- anyway lower than 1 ppm -- as to
allow equipment made from traditional materials to be
used, thus avoiding the problems connected with the
use of the usual neutralization techniques as
described above.
-- 215086
5.
Although this type of solid adsorbants is
normally used in order to remove acidic impurities
from incondensible process gases such as, e.g., in
dehydrochlorination of hydrogen coming from refinery
"reforming", they mere never used before for removing
acidic impurities from organic vapours. On the
contrary, it is surprising that such systems are
effective in removing hydrogen chloride from organic
vapours and Water, because these should effectively
compete with hydrogen chloride for the adsorption
sites. In fact, such systems are normally used for
adsorbing acidic impurities from incondensible gases
(air, and so forth), as, e.g., disclosed in Ullmanns:
"Encyklopddie der Technischen Chemie", Band 2, Volume
4, page 600.
The use of the method according to the present
invention is all the more surprising When one
considers that substantial water amounts (from 2% to
6% by Weight) are present in the vaporized stream
because water, in larger amounts than 0.1%-0.2% by
Weight, is known to inhibit adsorption of acids by
alumina, because Water competes with them for active
centres.
Therefore, the object of the present invention is
a method for removing the acidic impurities contained
in a vaporized stream composed by an alkanol, an alkyl
carbonate, Water and other organic products in minor
amounts, which method consists in sending said stream
to an adsorption reactor containing a solid adsorbent
bed comprising alumina or activated charcoal.
21~0~~8
6.
Solid adsorbent beds useful for the purpose of
the present invention are also those comprising
modified aluminas or modified activated charcoals.
Aluminas or modified aluminas useful for the
purpose of the present invention are those which are
available from the market, produced by Katalco
(I.C.I.), ALCOA (Aluminum Company of America), La
Roche Chemicals, Discovery Aluminas.
Specific examples of alumina or modified alumina
grades useful for the purpose of the present invention
are: Puraspec~R~ 2110, produced by Katalco, which is an
alumina modified with sodium salts Csodium aluminate);
A-203 CI~R~, manufactured by La Roche Chemicals, which
is a modified alumina with inorganic promoters; Cl
750~R~, produced by Discovery Aluminas, which is a
modified alumina with inorganic salts; Selexsorb~R~
SPCL, produced by ALCOA, which is a modified alumina
with inorganic promoters.
Activated charcoals or modified activated
charcoals useful for the purpose of the present
invention are those grades which are available from
the market, produced by Calgon Carbon Corporation,
American Norit, Degussa and 4lestvaco Corporation.
Specific examples of activated charcoals or
modified activated charcoal grades useful for the
purpose of the present invention are: Calgon Type
IVP~R~, which is a modified activated charcoal
impregnated with sodium hydroxide, produced by Calgon
Carbon Corporation; Darco~R~, which is an activated
charcoal produced by American Norit.
. ' . 2~~o~ss
7.
According to the present invention, aluminas and
activated charcoals are advantageously used in pellet
form, with an average size of the pellets comprised
within the range of from 2 mm to 4 mm of diameter.
The geometry of the adsorbent bed useful for the
purpose of the present invention, is the usual
geometry as used in industrial facilities for the
absorption of acidic impurities from incondensible
gases, or gas purification. For example, an adsorbent
bed can be used which is contained in a reactor in
which the ratio of reactor height to diameter is
comprised within the range of from 3 to 20. Said
adsorbent beds are capable of retaining amounts of
hydrogen chloride of up to 10%-14%, based on their
weight.
For the purpose of the present invention, the
vaporized stream containing the acidic impurities such
as, e.g., hydrogen chloride, is sent to the reactor
containing the solid adsorbant.
The above said vaporized stream can possibily
contain incondensible gases, such as, e.g., Hz, C0, C02
and NZ. In this case, the percent ratio of
incondensible gases to condensible vapours is
generally comprised within the range of from 0% to 90%
by volume, preferably of from 0% to 70% by volume.
The temperature and pressure conditions under
which the method is carried out, are not critical. In
general, the reactor containing the solid adsorbent
bed is kept at a temperature comprised within the
range of from 30°C to 150°C, preferably of from 50°C to
~1~0868
8.
110°C and under a pressure comprised within the range
of from 0.1 to 50 abs. atm, preferably of from 1 to 40
abs. atm, with the proviso that the selected pair of
values should be such as to prevent the vapours from
condensing under the selected operating conditions.
The contact times are advantageously comprised
within the range of from 0.3 to 30 seconds <as
calculated under standard conditions), to which values
of GHSV (Gas Hourly Space Velocity) correspond which
are comprised within the range of from 12000 to 120
hours, more preferably with contact times comprised
within the range of from 0.6 to 18 seconds, to which
GHSV values correspond which are comprised within the
range of from 6000 to 200 hours .
Necessary condition for method applicability is
the chemical inertness of vaporized substrates versus
the adsorbant. In this case, the removal of any types
of acidic impurities from any types of organics
streams, also containing Water, can be carried out by
using the method according to the present invention.
According to an embodiment of the present
invention, the above said method can be used in order
to remove the acidic impurities from vaporized streams
containing methanol, dimethyl carbonate, water and
still other organic species in minor amounts.
Preferred vaporized streams for the purpose of
the present invention are those having the following
composition (% by weight):
-- CH30H: from 45% to 70%;
-- DMC: from 25% to 50%;
215086
9.
-- HCI: from 5 to 1000 ppm;
-- H20: from 2X to 6Y; and
-- other organic byproducts: from 2X to 3Y.
The method according to the present invention
makes it possible the acidic impurities to be removed
down to lower levels than 1 ppm by Weight.
In the case when the method according to the
present invention is applied to DMC synthesis process,
the process stream sent as feed to the product CDMC)
purification/separation section results to be free
from hydrogen chloride, with the consequent
considerable advantage that for building said section,
normal materials can be used Without any corrosion
problems and financial burdens.
In order to better understand the present
invention and to practice it, some illustrative
examples are reported in the following, which shall
not be construed as being in anyway limitative of the
purview of the same invention.
EXAMPLE 1
The apparatus used in this example is illustrated
in the Figure.
Said apparatus is constituted by a peristaltic
pump which delivers the feed stream to a vaporizer
constituted by a glass tube coi l immersed in an of L
bath Which is at a suitable temperature for vaporizing
all feed, a preheater, the adsorption reactor
containing alumina or activated charcoal, and finally
a condenser With its relevant condensate collecting
flask.
210868
10.
All lines connecting the several equipment pieces
are of glass or teflon and are heated with an
electrical heating band, so as to prevent the feed
stream from getting condensed at cold spots.
The alumina containing reactor has a diameter of
15 mm and a variable height, ranging from 28 to 75 cm,
as a function of the amount of adsorbant which one
wishes to charge to it.
In Table 1, the results are reported which were
obtained as a function of the used types of alumina or
activated charcoal; the contact times, expressed as
GHSV Ch~~; the temperature of alumina or activated
charcoal containing adsorption reactor.
--- 2150868
11.
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Feed (% by weight):
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-- HCI: 5 ppm.
EXAMPLE 2
This example discloses a typical test of
adsorption of hydrogen chloride contained in a
vaporized feed which is caused to flow above an
alumina or activated charcoal bed. The apparatus used
for this purpose is the same apparatus as used in
Example 1, shown in the Figure.
The adsorption reactor is filled with PURASPEC~R~
2110 alumina in form of pellets of 5x8 mesh (42 ml,
34.3 g), and the reactor temperature is set at 80°C,
the temperature of the vaporizer is set at 100°C and
the preheater temperature is set at 80°C.
To the vaporizer, a stream of 210.4 g/h is fed
(GHSV of about 2800 h~) of a solution having the
ZO following composition (% by weight):
-- H20: 4.78%;
-- CH30H: 60.41%;
-- DMC: 34.82%; and
-- HCI: 5 ppm;
and 210 g/h of condensate is collected, having the
following composition (% by weight):
-- H20: 4.68%;
-- CH30H: 60.8%;
-- DMC: 34.52%; and
30 -- HCI: < 1 ppm.
-- - 21~O~~aB
13. '
The test is continued for longer than 50 hours
and the adsorption of hydrogen chloride was always
complete. Measurements of condensate conductivity
yielded values comprised Within the range of from 0.15
~cS to 0.3 ~.S throughout the test time.
EXAMPLE 3
The same apparatus is used as disclosed in
Example 1.
In this case the feed has the following
composition (X by Weight):
-- H2o: s%;
-- CH30H: 60%;
-- DMC: 35%; and
-- HCI: 500 ppm.
The purpose of this example is of verifying the
adsorption degree supplied by aluminas and activated
charcoals listed in following Table 2, as a function
of the contact time (GHSV), with the amounts of
adsorbed hydrogen chloride being expressed as X by
weight based on Weight on charged adsorbant.
The test results are reported in Table 2. Such
results clearly demonstrate that, although the
concentration of hydrogen chloride Was increased up to
500 ppm, all tested alumina and activated charcoal
grades completely adsorb the acid the concentration of
which, in fact, remains constantly lower than 1 ppm in
the condensates, until the adsorbed amounts are
reached which are reported in Table 2.
~150~68
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